Method for depositing metal

ABSTRACT

A method of depositing aluminum or other metals so that vias are more completely filled is disclosed. The wafer or substrate is preheated to a temperature of approximately 200° C. Then the wafer is placed in an ambient of approximately 350° C. while metal deposition commences. The resulting metal layer has a gradually increasing grain size and exhibits improved via filling. Also disclosed is a method and apparatus (involving cooling of support structures) for deposition of an anti-reflective coating to prevent rainbowing or spiking of the coating into the underlying metal.

This is a division of application Ser. No. 08/706,932 filed Sep. 3,1996, now U.S. Pat. No. 5,807,760, which is a continuation applicationof prior application Ser. No. 08/492,357 filed on Jun. 19, 1995 which acontinuation application of prior application Ser. No. 08/197,654 filedon Feb. 17, 1994 which is a continuation application of priorapplication Ser. No. 07/964,106 filed on Oct. 20, 1992 which is acontinuation application of prior application Ser. No. 07/629,925 filedon Dec. 19, 1990 abandoned.

TECHNICAL FIELD

The present invention relates to the manufacture of integrated circuits,and more particularly to methods for forming metal layers during thecourse of integrated circuit fabrication.

BACKGROUND OF THE INVENTION

Conventional integrated circuits manufacturing frequently includes theformation of active devices such as transistors upon an appropriatesubstrate. The active devices are next covered with a dielectricmaterial. Openings, often termed "windows" or "vias," are created in thedielectric. Next a conductive material, typically, a metal containingpredominantly, for example, aluminum (and its alloys, such as thosecontaining silicon and/or copper or both) is deposited in an argonatmosphere often by sputtering or physical wafer depostion, or tungstenby chemical wafer depostion, in a layer over the dielectric and withinthe opening.

An anti-reflective coating (ARC) (usually silicon) is deposited over theconductor to facilitate lithography. Then the conductor is subsequentlypatterned to form conductive runners between individual devices.

It is important that the anti-reflective coating thickness be maintainedcomparatively uniform so that spurious reflections are not created--thusinterfering with subsequent lithography. It is also important thatwhatever conductive material is deposited, the opening be adequatelyfilled to insure good electrical contact between the underlying deviceand the runner (and ultimately other devices in the circuit).

Aluminum is often used as a material for conductive runners. It has beenfound that the performance of aluminum runners in integrated circuitsdepends somewhat upon the conditions under which the aluminum runnersare formed.

Various factors may affect the deposition of aluminum layers. Some ofthese factors are discussed below. In recent years, stress-induced voidshave been reported as a major mode of failure for aluminum lines.Stress-induced voiding is due to tensile stresses generated in thealuminum lines during the cooling that follows deposition of an oxide ora nitride passivation layer. It has been found that increasing the metaldeposition temperature helps to reduce problems associated withstress-induced voiding.

However, the higher deposition temperature can give rise to a newproblem, namely the pull-back of deposited aluminum layers in vias andwindows. The pull-back phenomenon (schematically illustrated in FIG. 1)is exacerbated as the deposition temperature is increased.

As mentioned before, after the aluminum is deposited, an ARC is formedupon the upper surface of the aluminum. The ARC is generally boron-dopedamorphous silicon. Should the aluminum/ARC combination be heated, thesilicon ARC tends to migrate into the aluminum. Consequently, thethickness of the ARC changes. The change in thickness of the ARC isevidenced by "rainbowing," i.e., multicolored reflections emanating fromthe variable thickness silicon ARC. However, maintenance of a relativelyconstant ARC is important to the success of the following lithographysteps in achieving uniform linewidths.

SUMMARY OF THE INVENTION

The present invention helps to address both the via-filling problem andthe rainbowing-spiking problem. The present invention helps to insureadequate via filling without exacerbating stress-induced voiding.Illustratively, the invention includes an integrated circuit with adielectric layer having an opening. The opening is substantially filledwith a metal which has a grain size that gradually increases through theopening.

Another illustrative embodiment includes a fabrication method fordepositing a metal which includes preheating a substrate to a firsttemperature and then exposing the substrate to an ambient environment ata second higher temperature and depositing the metal as the temperatureof the substrate rises toward the second temperature. The illustrativeprocess permits the formation of a metal with a gradually increasinggrain size. Metal thus deposited in a via will tend to adequately fillthe via and not tend to exhibit pull-back.

Concerning the rainbowing problem in the ARC, the present inventionhelps to prevent rainbowing by providing a support structure maintainedat a constant temperature during deposition of the ARC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views depicting the advantages of anillustrative embodiment of the present invention.

FIG. 3 is a partial perspective, partial cross-sectional view of anotherillustrative embodiment of the present invention.

DETAILED DESCRIPTION

An illustration of the pull-back phenomenon is provided in FIG. 1.Reference numeral 11 denotes a substrate which, illustratively, may besilicon, or epitaxial silicon, with or without dopants. (Referencenumeral 11 may also denote a conductive runner.) Dielectric 15 generallycovers substrate 11. Opening 17, which may be a via or window, exists indielectric 15. Conductive layer 13, which may be illustrativelyaluminum, or a mixture of aluminum with copper and/or silicon added, hasbeen deposited upon dielectric 15. It is desired that conductor 13 fillvia or window 17. However, as can be seen from the figure, theas-deposited conductive film 13 does not completely fill via or window17. A portion 21 of dielectric 15 together with a portion 19 ofsubstrate 11 is not covered by conductive layer 13. The pull-backphenomenon prevents the formation of good electrical contact betweenrunners formed from conductive layer 13 and substrate 11.

It is known that the grain size of aluminum (and other metals) increaseswith increasing temperature. As the temperature at which the aluminum isdeposited increases, the initial nuclei tend to be large. If the nucleicoalesce near the top of the vias or windows such as 17, they may tendto shadow the walls and prevent further deposition. In FIG. 1,coalescence of large nuclei near the point designated by referencenumeral 22 may inhibit conductor coverage on surfaces 19 and 21. Theincomplete filling is termed "pull-back."

The publication, Pramanik et al., Aluminum Metallization for ULSI, SolidState Technology, pp. 73-79, March 1990, suggests that the pull-backproblem may be solved by two-step deposition procedure in which a thinnucleating layer of aluminum is first deposited at a low temperature andthen the remainder of the film is deposited at a higher temperature.However, applicants' investigations have shown that when a two-stepdeposition process is employed, the pull-back problem still persists. Itshould be noted that the Pramanik et al. technique requires that thesubstrate be brought into thermal equilibrium with its environment at afixed first temperature after which a first deposition step isperformed. Then the wafer is brought into thermal equilibrium with asecond, higher temperature, environment and a second depositionperformed. Two discrete deposition steps are performed after the waferhas twice attained thermal equilibrium with its environment.

Applicants have discovered that the pull-back problem may be alleviatedby depositing metal continuously as the wafer temperature rises,approaching a second thermal equilibrium with its environment. Thecontinuous deposition, formed as the wafer temperature rises, provides ametallic layer in which the grain size increases very gradually, thus,helping to prevent pull-back and improving film quality.

The inventive process may be practiced in a sputter-depositionapparatus, such as the models 3180 and 3190 manufactured by VarianAssociates, Inc. Other apparatus such as physical vapor depositionapparatus may also be used. Initially the wafer is subjected to anambient environment at a temperature of between 150° C. and 200° C.,with approximately 200° C. being preferable. The wafer is allowed tocome into thermal equilibrium with the ambient. In a single-chambermulti-station Varian machine, the preheating may be accomplished in oneof three stations. Next, the wafer is transferred (without breakingvacuum) to another station in which the heater temperature is between350° C. and 400° C., preferably approximately 350° C. The wafer is notallowed to come to thermal equilibrium with the heater. Instead, sputterdeposition of aluminum and/or aluminum alloys containing silicon and/orcopper or both (typically approximately 0-2% silicon and 0-4% copper) isbegun. After approximately 50 seconds, a layer of approximately 10000 Åis deposited. If longer deposition times are used, thicker layers willresult. As the temperature of the wafer rises to approach thetemperature of the ambient environment, the average grain size of thedeposited aluminum tends to become larger. The initially small grainsize tends to produce a layer which covers the exposed surfaces ofdielectric 15 in FIG. 1 including surfaces 21 and 19. The subsequentlyformed larger grain sizes complete filling of the opening 17 thus,producing a filled via somewhat similar to that depicted in FIG. 2. Theresulting film exhibits improved via filling and satisfactory resistanceto stress-induced voiding.

Should it be desired, a layer of titanium, titanium nitride, or tungstenmay be deposited prior to the aluminum deposition. Deposition of theseother materials may be accomplished at a low temperature. Thus the"preheating" step described above, may be advantageously used, ifdesired, to deposit an additional metallic layer beneath the aluminumlayer.

The inventive technique may be applied to all metals which are formed bysputtering or chemical vapor deposition, such as aluminum, tungsten,molybdenum, and copper, together with composites rich in thesematerials.

The inventive technique is applicable to multi-chambered depositionapparatus such as a cluster tool also. It is necessary to provide forthe ramped heating of the wafer in the deposition chamber.

Turning to the rainbowing problem, applicants have noted that duringproduction processing, when a silicon ARC is sputter-deposited upon analuminum layer, that the silicon may migrate into the aluminum, thuscausing the rainbowing phenomenon mentioned before. Rainbowing ormigration may not occur on the first few wafers of a production lot.However, rainbowing or migration are often observed after several wafershave been processed. The reason for the occurrence is that the wafersupport apparatus is heated during the sputter deposition process. Thefirst wafers which undergo processing in a comparatively cold machine donot exhibit rainbowing. As the wafer support apparatus heats up duringsputter deposition upon several wafers, heat is transferred tosubsequent wafers, thus inducing rainbowing or spiking.

Depicted in FIG. 3 is a portion of the apparatus commonly used tosupport a wafer during deposition of materials such as by sputtering.Wafer 21 is supported above block 23 by a ring and clips not shown.However, outer edge 25 of wafer 21 contacts lip 27 of block 23.

An inert gas such as argon flows through holes 29 and contacts most ofthe under side of wafer 21. The gas, which may be heated, flows inthrough pipe 31 whence it is ducted by capillaries (not illustrated) toholes 29. Since the gas is admitted only through holes 29 near the edgeof the wafer 21, thermal gradients can be created across the wafer. Theexistence of thermal gradients means that there may exist favorableconditions for deposition on one portion of the wafer and less favorabledeposition conditions on another part of the wafer.

Furthermore, a thermal convection process frequently occurs involvingthe under side 33 of wafer 21, the inert gas, and the upper surface 35of block 23. (Recall, as mentioned before, that the upper surface 35 ofblock 23 does not contact the under side 33 of wafer 21.)

The thermal convection process mentioned above becomes particularlyimportant, for example, after the deposition of aluminum such as layer17 in FIG. 2 during the sputter deposition of an anti-reflectivecoating, which is, typically, boron doped amorphous silicon.

During the production of integrated circuits, a steady stream of wafersis placed into an apparatus such as that shown in FIG. 3 for ARC onaluminum. Inevitably the temperature of block 23 rises. Consequently,wafers which are processed first experience a temperature environmentwhich is lower than wafers processed an hour or so later. Theselater-processed wafers experience a higher temperature because ofconvection between the hot block 23 and the underside 33 of the wafer.

The increased temperature of the block may induce thermal gradients inthe wafer which may affect aluminum deposition, as mentioned before. Theincreased temperature of the wafer causes the ARC to migrate into theunderlying aluminum.

While some practitioners employ a separate cooling step before ARCdeposition, the separate cooling step is ineffective in alleviating theproblem of block-heating induced temperature increases during ARCdeposition.

Applicants have solved the problem of block heating by incorporating acooling coil 37 within block 23 and in proximity to surface 35 of block23. The coil (through which a variety of gaseous and liquid coolantsincluding water may be flowed) helps to maintain the temperature ofsurface 35 relatively constant even during long production runs in whichit is continually exposed to the heat generated by sputtering processes.The maintenance of a relatively constant temperature on surface 35 helpsto:

a.) improve the previously-discussed aluminum deposition process byreducing or eliminating thermal gradients across wafer 21 and helping toinsure a constant temperature for all wafers;

b.) improve the subsequent deposition of anti-reflective coatings byinsuring that each wafer in the production lot sees the sametemperature, thus helping to eliminate silicon migration and rainbowing.

We claim:
 1. An apparatus for sputter deposition of materials upon asubstrate comprising:a block for supporting said substrate, said blockhaving:a raised edge surrounding a flat portion, said raised edgeproviding support for the periphery of said substrate, there being aspace between said flat portion and said substrate, and a base, saidflat portion and said base forming a chamber; and a tube disposed insaid chamber, said tube cooling the block and containing a fluidcoolant, said tube being proximate to said flat portion of said block.2. The apparatus according to claim 1 wherein the tube forms a coil. 3.The apparatus according to claim 1 wherein the flat portion has aperiphery and holes formed at the periphery.